The polyester films have excellent in strengths, dimensional stability, and so on, and have widely been used for magnetic recording media, capacitors, packages and printing materials. Video tapes, audio tapes, computer tapes, and so on, are widely known as the magnetic recording media using the polyester films as supports (base films).
High density recordings on the magnetic recording media are progressed in recent years, accompanied by the formation of thin and flat base films. However, it is difficult to roll a thin flat film in a roll-like shape in a good rolled appearance. Even when the thickness of a film is slightly uneven, the unevenness of the film is accumulated, when rolled in the roll-like shape. Consequently, the thin portion of film is deformed into a wrinkle-like shape, and the thick portion of the film is extended and forms slacks, when the film is unrolled, whereby troubles are caused when the film is subjected to a processing such as a coating processing or a vacuum-deposition processing.
In order to solve the problems, various techniques such as the improvement in the surface characteristics of a film [JP-A 59-95116 (hereafter, JP-A means Japanese unexamined patent publication), JP-A 59-171623, JP-A 2-194924, JP-A 3-207727, and so on], the reduction of thickness unevenness (JP-A 48-43772, JP-A 52-47070, JP-A 54-56674, JP-A 1-95025, JP-A 1-295822, and so on), or the dispersion of the thickness unevenness into the width direction by oscillation (JP-A 36-22875, JP-A 39-14534, and so on] have been proposed.